One of the growth areas for mobile network operators and content providers is the provision of ringtones, wallpapers and other multimedia content for mobile telephones and devices. There is a tension between the needs of mobile network operators and device manufacturers to retain control over some aspects of the device user interfaces for branding purposes and the needs of users to customize and modify the appearance of their devices to suit their own needs. The sophisticated software required to provide the desired flexibility and customization is also in tension with the limited processing power and data storage capacity of typical mobile devices. The present invention seeks to mitigate these problems.